Learn Linux Shell Scripting - Fundamentals of Bash 4.4

Input checking

Closely related to setting default values with parameter expansion, we can also use parameter expansion to display an error if a variable is null or empty. Up until now, we've done this by implementing if-then logic within our scripts. While this is an excellent and flexible solution, it is a little verbose—especially if the only thing you're interested in is the user supplying the parameters.

Let's create a new version of our previous example: this one does not supply default values, but will alert the user if positional arguments are missing.

We'll use the following syntax:

${parameter:?word} Display Error if Null or Unset. If parameter is null or unset, the expansion of word (or a message to that effect if word is not present) ...